Assessing the Firing Properties of the Electrically Stimulated Auditory Nerve using a Convolution Model

The electrically evoked compound action potential (eCAP) is a routinely performed electrophysiological measure of the auditory nerve in cochlear implant users. Using a convolution model of the eCAP, additional information about the firing properties of the auditory nerve can be obtained. In this study, eCAPs for a monopolar biphasic pulse were analysed in two groups: acutely implanted normal-hearing and deafened guinea pigs, and human cochlear implant recipients. The estimated firing probability of the auditory nerve could be parameterised for most eCAPs by a bimodal probability distribution consisting of two Gaussian distributions with an average latency difference of 0.4-0.5 ms. The ratio of the late and early Gaussian distributions increased with neural degeneration in the guinea pig and this ratio decreased with stimulation intensity. This decrease with intensity was also observed in humans. The latency of the early Gaussian distribution decreased with neural degeneration in the guinea pig. Indirectly, this was observed in humans as well, assuming that the cochlear base exhibits more neural degeneration than the apex. Differences between guinea pigs and humans were observed, among other parameters, in the width of the early component: very robust in guinea pig, and dependent on stimulation intensity and cochlear region in humans. We conclude that the deconvolution of the eCAP is a valuable addition to existing eCAP analyses, in particular in revealing two separate firing components.